1. Field of the Invention
The present invention relates to multi-jackbolt threaded tensioner assemblies and, more particularly, to a double nut tensioner assembly for pre-loading threaded tie rod members.
2. Description of the Prior Art
Hydro-forming is a well known metal working process that uses pressurized fluid to deform a closed channel workpiece, such as a tubular member, outwardly into conformance with a die cavity having a desired shape. As illustrated in FIG. 1, the typical hydro-forming press apparatus includes a frame having lower and upper platens 102 and 104 respectively, for receiving a hydro-forming die (not shown) therebetween having lower and upper die sections that are supported thereon for relative movement between opened and closed positions. The die sections have cooperating recesses formed therein that together define a die cavity having a shape corresponding to a desired final shape for the workpiece. When moved to the opened position, the die sections are spaced apart from one another to allow a workpiece to be inserted within or removed from the die cavity. When moved to the closed position, the die sections are disposed adjacent to one another so as to enclose the workpiece within the die cavity. The workpiece is then filled with a fluid, typically a relatively incompressible liquid such as water. The pressure of the fluid within the workpiece is increased to such a magnitude that the workpiece is expanded outwardly into conformance with the die cavity. As a result, the workpiece is deformed or expanded into the desired final shape. Hydro-forming is an advantageous process for forming vehicle frame components and other structures because it can quickly deform a workpiece into a desired complex shape.
The upper platen 104 and the lower platen 102 are connected together by a set of vertically extending compression tubes 106 (only one is illustrated in FIG. 1). The compression tubes 106 are generally hollow and cylindrical in shape. A tie rod 112 extends through each of the compression tubes 106 from the lower platen 102 to the upper platen 104. Each of the tie rods 112 is a generally solid cylindrical member having an upper end portion 114 that extends above the upper platen 104 and a lower end portion 116 that extends below the lower platen 102.
To maintain the die sections together during the hydro-forming process, a mechanical clamping device is usually provided. The mechanical clamping device mechanically engages the die sections to prevent them from moving apart from one another during the hydro-forming process. Usually the mechanical clamping device includes two or more tensioner assemblies 110 illustrated in detail in FIG. 2.
In the illustrated embodiment, each of the tensioner assemblies 110 includes the tie rod 112 having threaded end portions 114 and 116, a thrust member 118 concentrically positioned about the tie rod 112 and placed on the upper platen 104 and a nut member in the form of a nut member 120 or similar retaining devices, threaded onto the threaded end portions 114 and 116 of the tie rods 112. The nut member 120 is further provided with a plurality of threaded holes at circumferentially spaced apart locations extending substantially parallel to and spaced from the tie rod 112. As further illustrated in FIG. 2, the tensioner assembly 110 is further provided with a set of jackbolts 123 adapted to threadably engage the holes in the nut member 120. The jackbolts 123 are provided to extend from the holes in the nut member 120 to exert pressure against the thrust ring 118 through torquing the jackbolts 123 for applying compressive reaction forces on the thrust ring 18 in order to pre-stress the rod member 112 and the compression tube 106.
In operation of the hydro-forming press apparatus, the tie rod 112 is subject to a tension stress variation due to a pre-load thereof and a pressure fluctuation within the die cavity of the hydro-forming press apparatus. As a result, the rods of the hydro-forming press apparatuses may crack prematurely due to the stress fluctuation in the tie rods.
Minimum and maximum stresses at the threads engaged with the tensioner assembly 110 determine a fatigue strength of the tie rod 112. More specifically, the effective parameters are the alternating stress range that is the difference of the maximum and minimum stresses and the mean stress that is the average of the maximum and minimum stresses. The smaller the alternating stress range and/or the mean stress, the higher the fatigue strength.
Accordingly, it is the intent of this invention to overcome these shortcomings of the prior art, such as low fatigue strength of the tie-rods subject to tension stress variation.